Field of the Invention
Attempts to improve pharmacological effects and to reduce side effects have been made since long ago. As one of recent attempts, a drug delivery system (DDS) may be mentioned. DDS is an attempt to let a drug transmigrate to a necessary site preferably selectively for a necessary period of time, so that the pharmacological effects be improved, and general side effects be substantially reduced. Various types are available as carriers for DDS. Among them, lyposome and lipid microspheres may be mentioned as typical examples. Lyposome is prepared by dissolving a natural lipid such as lecithin or cholesterol in an organic solvent, then dispersing it in water by e.g. ultrasonic treatment, and sealing a drug therein. Whereas, lipid microspheres are prepared by suspending soybean oil in water together with lecithin, whereby lecithin constitutes the surface and a drug is sealed in. In each case, the drug is sealed in primarily by physical bond. Lyposome has a drawback that the stability is poor. Whereas, in the case of lipid microspheres, the drug to be sealed in must be lipid-soluble, and it is further required to employ a special apparatus for the production.